1. Technical Field
The present disclosure relates to a method for making a carbon nanotube structure.
2. Description of Related Art
Carbon nanotubes are tubules of carbon generally having a diameter of about 0.5 to about 100 nanometers, and composed of a number of coaxial cylinders of graphite sheets. Carbon nanotubes have interesting and potentially useful thermal, electrical, and mechanical properties, and have recently attracted a great deal of attention for use in different applications such as field emitters, gas storage and separation, chemical sensors, and high strength composites.
However, the main obstacle to actual application of carbon nanotubes is their difficulty to process due to the powder form of the carbon nanotube products. Therefore, forming separate and tiny carbon nanotubes into manipulable carbon nanotube structures is necessary. The manipulable carbon nanotube structure can be a carbon nanotube film, a carbon nanotube wire, or a carbon nanotube cable.
Recently, as disclosed by U.S. Pat. No. 7,045,108 to Jiang et al., an untwisted carbon nanotube wire has been fabricated by drawing from a carbon nanotube array. The untwisted carbon nanotube wire is free standing and includes a plurality of carbon nanotubes joined end-to-end by van der Waals attractive force therebetween. The carbon nanotubes in the untwisted carbon nanotube wire are substantially aligned along the length direction of the carbon nanotube wire. Thus, this carbon nanotube wire has good properties, such as thermal and electrical conductivities, along the direction of the aligned carbon nanotubes. Therefore, the carbon nanotube wire can be used in many different fields. Similarly, a carbon nanotube film can be fabricated by drawing a film from the carbon nanotube array.
However, sizes of the carbon nanotube wire and carbon nanotube film, which are directly drawn from the carbon nanotube arrays, are restricted by the sizes of the carbon nanotube arrays. The carbon nanotube array is grown on a flat surface of a silicon wafer by using a chemical vapor deposition (CVD) method. More specifically, a catalyst film is deposited on the flat surface of the silicon wafer, and then the silicon wafer is disposed and heated in a tube furnace. Carbon source gas and protective gas are introduced into the tube furnace and the carbon source gas is pyrolyzed by an action of the catalyst film at elevated temperature to grow the carbon nanotube array on the flat surface of the silicon wafer. During the growing, the inner gas pressure of the tube furnace is less than the atmosphere pressure outside the tube furnace. Therefore, the sidewall of the tube furnace has to bear an inward pressure difference applied thereon. When the tube furnace with a diameter of about 10 inches and a length of about 2 meters has the inner gas pressure of about 10 torr, the pressure difference between inside and outside of the tube furnace is about 50,000 Newton. However, if the diameter of the tube furnace increases to 40 inches, the pressure difference could reach to about 200,000 Newton. Further, as the increase of the diameter of the tube furnace, the curvature of the sidewall of the tube furnace decreases, and thus weakens the support of the sidewall. Therefore, as the increase of the diameter of the tube furnace, a large inward pressure difference may cause a damage of the tube furnace.
Accordingly, the tube furnace with a larger diameter can hardly be achieved. The conventional tube furnace for growing the carbon nanotube array has a diameter of about 10 inches. Therefore, the silicon wafer disposed inside the tube furnace should have a diameter less than 10 inches, such as 8 inches. An original carbon nanotube film directly drawn from the carbon nanotube array grown on that 8-inch-silicon-wafer has a width restricted to 8 inches. The diameter of the carbon nanotube wire is also restricted.
What is needed, therefore, is to provide a method for making a carbon nanotube structure having relatively large size